Method for reversibly storing hydrogen on the basis of alkali metals and aluminum

ABSTRACT

A method for reversibly storing hydrogen, characterized in that reversible hydrogen storage materials are used which contain mixtures of aluminum metal with alkali metals and/or alkali metal hydrides and transition metal and/or rare-earth metal catalysts.

This application is a 371 of PCT/EP01/02363, filed on Mar. 2, 2001.

The present invention relates to a method for reversibly storinghydrogen using alkali metals or their hydrides and aluminum metal ashydrogen storage materials and doping with transition metal catalysts.

According to the patent application PCT/WO 97/03919 of theStudiengesellschaft Kohle mbH (SGK), a method for reversibly storinghydrogen is known using the alkali metal alanates of general formula M¹_(p(1−x))M² _(px)AlH_(3+p) (1) as storage materials, where M¹=Na, K;M²=Li, K; 0≦x≦˜0.8; 1≦p≦3. To improve the hydrogenation/dehydrogenationkinetics, the alkali metal alanates are doped with transition metalcompounds in catalytic amounts. In particular, the alanates NaAlH₄,Na₃AlH₆ and Na₂LiAlH₆ are employed.

The drawbacks of the previous SGK method are that the preparation andpurification of commercial sodium alanate, the preparation of Na₃AlH₆ orNa₂LiAlH₆ and the subsequent doping in organic solvents are relativelycomplicated on a preparative level and in most cases require the use ofsolvents which are highly volatile and highly inflammable (ether,pentane) or tend to form peroxides (ether, THF).

Surprisingly, it has now been found that instead of using thetransition-metal doped alkali metal alanates as hydrogen storagematerials, the starting materials used for their preparation in the formof alkali metal hydrides or alkali metals (especially NaH and Na), Alpowder and doping agents can be employed. The alanates formed in onehydrogenation step from such starting materials are immediatelyfunctioning as H₂ storage materials and have improved storage propertiesas compared to PCT/WO 97/03919.

Methods for the preparation of alkali metal alanates from alkali metalhydrides (or alkali metals), aluminum and hydrogen are known. A surveyof the methods for the preparation of NaAlH₄, Na₃AlH₆ and Na₂LiAlH₆ canbe found in J. Alloys & Compounds, 298 (2000) 125-134. Thus, accordingto the German Patent Specification 1 136 987 (1962), Na and Li alanatescan be prepared by reacting the corresponding alkali metal hydrides (oralkali metals) and aluminum in ethers, amines and aliphatic or aromatichydrocarbons, optionally in the presence of catalytic amounts oforganoaluminum compounds, with hydrogen under pressure. The U.S. Pat.No. 3,138,433 (1964) describes, inter alia, a method for the preparationof NaAlH₄ from NaH, Al and hydrogen under pressure in THF using Ti, Zr,Hf and Th tetrahalides as catalysts; however, in the only patent examplecontained therein, a maximum of 21.8% is stated as the yield of NaAlH₄.The direct synthesis of Na₃AlH₆ can be successfully performed with 98%yield according to Inorg. Chem. 5 (1966) 1615 by reacting Na andactivated Al powder in diglyme in the presence of Et₃Al with hydrogenunder pressure (350 bar). The synthesis of NaAlH4 from the elements Na,Al and H₂ is also possible in the absence of organic solvents accordingto Dokl. Akad. Nauk SSSR 215 (1974) 1369, Engl. 256, by performing theprocess in the molten state (≧175 bar, <280° C.). The alkali metalalanates prepared according to the methods mentioned were not consideredfor purposes of hydrogen storage.

In contrast, the preparation of the storage material according to thepresent invention is very simple, completely dispensing with organicsolvents. The aluminum powder used according to the present method ischeaper and more easily handled than sodium alanate, which waspreviously employed. It was particularly surprising that thehydrogenation of aluminum in the presence of alkali metals or metalhydrides can be successfully performed at temperatures which areconsiderably below the melting points of the metal/metal hydride eductsinvolved and the metal alanate products, i.e., in a solid state (incontrast to the above referenced direct synthesis according to Dymova etal., Dokl. Akad. Nauk SSSR 215 (1974) 1369, Engl. 256 “Direct Synthesisof Alkali Metal Aluminium Hydrides in the Melt”).

According to the present invention, for example, aluminum powder ismixed with powdered sodium hydride and admixed with catalytic amounts oftitanium tetrabutylate. The composition thus obtained can be useddirectly as a reversible hydrogen storage material, When Al and NaH areemployed at a molar ratio 1:1, NaAlH₄ is obtained in the hydrogenation,whereas a molar ratio of 1:3 yields Na₃AlH₆ after hydrogenation.

Another particular advantage of the present method for reversiblystoring hydrogen is that the desorption and absorption kinetics could besignificantly improved by facilitating the previously known methodaccording to PCT/WO 97/03919.

In FIG. 1, the hydrogen desorption at 160° C. under normal pressureaccording to the present invention is shown as compared to thepreviously known method of the Studiengesellschaft Kohle. According tothe previous method, a complete discharge of the storage material takesabout 10 h, while desorption according to the present invention onlytakes about 1 h.

FIG. 2 shows the course of hydrogenation of a hydrogenation cycleaccording to the previous method at 170° C. and according to the presentmethod at 118° C. and illustrates the significantly increased activity.

In FIG. 3, 33 hydrogenation/dehydrogenation cycles of a materialobtained according to the present method are recorded, demonstrating thereversible character of the new materials.

A typical storage material preparation according to the presentinvention consists in intensely stirring aluminum powder, untreated orafter brief heating under vacuum to about 200° C., with finely powderedsodium hydride under a protective gas (e.g., argon). Subsequently,catalytic amounts of titanium tetra-n-butylate (from 0.1 to 10 molepercent, based on aluminum, preferably from 1 to 5 mole percent) areadded dropwise with stirring (or optionally under milling). In this way,a gray and slightly tacky, but still pourable powdery mixture isobtained, which is then charged into an autoclave. At first,hydrogenation is performed under pressures of between 5 and 150 bar andtemperatures of between 20 and 200° C. Subsequently, dehydrogenation isperformed against elevated pressure or normal pressure at temperaturesof between 50 and 250° C., thus periodically cycling in a suitablepressure installation (Example 1).

In order to achieve as good as possible hydrogenation kinetics and highstorage capacities, the aluminum is preferably employed in the form offine grindings (see Examples 1 and 5: the surface areas according to BETmeasurement are 12.2 and 2.0 m²/g, respectively).

In a modification of the storage material preparation described, thealuminum employed may optionally be preactivated by milling,ultrasonication or chemical activation. Instead of sodium hydride orsodium, other alkali metal hydrides or alkali metals (especially Li andK) may also be used singly or in combinations. The molar ratio betweenaluminum and alkali metal can be varied between 1:0.3 and 1:5. Ifaluminum and Na or NaH are employed at a molar ratio of about 1:1 orabout 1:3, NaAlH₄ or Na₃AlH₆, respectively, are obtained in thehydrogenation. Optionally, the alkali metals of their hydrides may bepretreated by milling or ultrasonication prior to being used. Ascatalysts, transition metals or transition metal compounds or alloys ofPeriodic Table groups 3 to 11 and of the rare-earth metals which may bebonded to elements of groups 14 to 17 or to hydrogen are used singly orin combinations. The transition metal or rare-earth metal compounds arepreferably employed in the form of halides, hydrides, alcoholates,amides or organometallic compounds. Particularly preferred are halides,alcoholates and organometallic compounds of titanium, zirconium and therare-earth metals.

The advances of the present method over the previous SGK method (PCT/WO97/03919) reside in the following improvements:

the educts are readily available commercially;

lower process costs;

significantly facilitated preparation of the storage material;

no organic solvents are used;

significantly improved hydrogenation and dehydrogenation kinetics.

The invention is further illustrated by the following Examples withoutbeing limited thereto. All experiments were performed in a protectivegas atmosphere, e.g., argon.

EXAMPLE 1 Mixture of Al and NaH Powders Doped with TitaniumTetra-n-butylate as Reversible Hydrogen Storage Material; 33 Cycle Test

The aluminum employed was Al grindings (Lunasol) supplied by theFrankfurter Bronzefarben-und Blattmetallfabrik Julius Schopflocher AGand having a surface area of 12.2 m²/g (according to BET measurement).

The NaH was finely powdered in a glass bead mill. The aluminum powderwas briefly heated to 200° C. at 0.1 mbar (aluminum content according toan elemental analysis performed by the company H. Kolbe of Mülheim a.d.Ruhr: 91.7% by weight).

Under argon, 753 mg (31.4 mmol) of the powderized sodium hydride wasintensely mixed with 980 mg (33.3 mmol) of the vacuum-heated aluminumgrindings by stirring with a magnetic stirring bar. Then, 0.21 ml (0.62mmol=1.9 mole percent, based on Al) of titanium tetra-n-butylate wasslowly added to the stirred powder from a fine dropping syringe, andstirring was continued for a short time. 1850 mg of the gray andslightly tacky, but still pourable powdery material obtained was chargedinto an autoclave (volume about 40 ml) with a glass lining. Theautoclave was equipped with an interior temperature sensor, electricheating with a ramping function, electric pressure transducer and amultichannel recorder. In order to test the suitability of the materialas a reversible hydrogen storage material, it was subjected to a seriesof 33 hydrogenation/dehydrogenation cycles (cycle test) (see Table 1).The cycle test was performed in a so-called open system, i.e., for eachhydrogenation, fresh hydrogen (99.9%) was taken from a hydrogen pressuretank, and for each dehydrogenation, hydrogen was desorbed against normalpressure.

TABLE 1 Cycle test (Example 1) Final % by # of Temp. pressure^(a)) Timeweight cycles [° C.] [bar] [h] of H₂  1 165 133 24 3.03  2 135 135 8.73.41  3 135 137 13 3.77  4 103  89 20 3.72  5 135 125 9.8 3.88  6 116103 4.6 3.81  7 118   127^(b)) 7 3.88  8 120 127 7.3 3.96^(c))  9 120126 7.5 3.99 10 120 121 4.5 3.95 11 120 128 4 3.91 12 120 129 4 3.87 13120 125 4.3 3.83 14 120 132 4.6 3.82 15 120 130 3.8 3.76 16 120 128 9.43.78 17 120 131 4.5 3.81 18 120 132 4.5 3.78 19 120 129 4.6 3.81 20 120130 5 3.76 21 120 132 5 3.79 22 120 128 5.5 3.77 23 120 130 5.5 3.68 24120 127 6.5 3.67 25 120 129 5.3 3.68 26 120 132 5.7 3.64 27 120 131 11.53.67 28 120 108 7.5 3.60 29 120 134 5.3 3.57 30 120 96 7.5 3.55 31 120132 5.5 3.56 32 120 133 5.5 3.54 33 120 123 5.8 3.56 ^(a))Forestablishing the initial pressure, about 4 bar per percent by weight ofH₂ must be added to the final pressure. ^(b))See FIG. 2 ^(c))See FIG. 1.

Hydrogenation: The hydrogenations were performed at temperatures ofbetween 103 and 165° C., mostly as about 120° C., under a decreasinghydrogen pressure in an autoclave (see FIG. 2, 7th cycle).

Dehydrogenation: The sample was quickly heated from room temperature to160° C. and kept constant at this temperature until the end of hydrogenevolution. The time course of hydrogen evolution was recorded with theaid of an automatic gas burette (Chem. Ing. Tech. 55 (1983) p. 156)together with the interior temperature of the sample. FIG. 1 shows thecourse of dehydrogenation (8th cycle, 3.96% by weight of H₂) as comparedto the prior art.

The dependence of the hydrogen storage capacity (measured by the amountof H₂ released during dehydrogenation) on the number of cycles is shownin FIG. 3.

After a total of 34 hydrogenation cycles, the storage material in thehydrogenated form was removed from the autoclave and examined byinfrared spectroscopy. The IR spectrum shows AlH₄ and AlH₆ bands inaddition to weak CH and C—O bands (alcoholate groups).

EXAMPLE 2 Mixture of Al and NaH Powders Doped with TitaniumTetra-n-butylate as Reversible Hydrogen Storage Material Using UntreatedAluminum Grindings

The preparation of storage material was effected by analogy with Example1, but employing untreated commercial aluminum rather than the productheated under vacuum. The material was examined in 7 cycles and reached astorage capacity of 3.7% by weight of H₂ in the 3rd hydrogenation cycleand 3.6% by weight of H₂ in the 7th hydrogenation step.

EXAMPLE 3 Mixture of Al and NaH Powders Doped with β-TiCl₃ as ReversibleHydrogen Storage Material

The preparation of storage material was effected by analogy with Example1, except that the aluminum grindings were not heated under vacuum, butmilled mechanically in a glass bead mill prior to use. Instead ofTi(OBu)₄, 2 mole percent of β-TiCl₃ was used for doping. The materialwas cycled and reached a capacity of 2.5% by weight of H₂ in the 1sthydrogenation and 2.9% by weight of H₂ in the 5th hydrogenation step (at135° C./about 140 bar).

EXAMPLE 4 Mixture of Al and NaH Powders [Molar Ratio=1:2.9] Doped withTitanium Tetra-n-butylate for the Preparation of Na₃AlH₆ as ReversibleHydrogen Storage Material

The preparation of storage material was effected by analogy with Example1, except that the aluminum grindings were not heated under vacuum, butmilled mechanically in a glass bead mill prior to use. The molar ratiobetween aluminum and sodium hydride was 1:2.9. The material reached acapacity of 2.2% by weight of H₂ in the 1st hydrogenation and 1.5% byweight of H₂ in the 5th hydrogenation step (at 117° C./35 bar).

EXAMPLE 5 Mixture of Al and NaH Powders Doped with TitaniumTetra-n-butylate as Reversible Hydrogen Storage Material Using SphericalAl Powder of about 20 μm

The preparation of storage material was effected by analogy with Example2, but employing spherical Al powder (about 20 μm) supplied by Aldrich(surface area of 2.0 m²/g according to BET measurement) in untreatedform instead of the Al grindings. The material reached a capacity of0.9% by weight of H₂ in the 1st hydrogenation (165° C./150 bar) and 1.5%by weight of H₂ in the 2nd hydrogenation step (165 to 182° C./150 bar).

What is claimed is:
 1. A method for reversibly storing hydrogen,catalysts said method comprising a) hydrogenerating a mixture ofaluminum metal with alkali metals and/or alkali metal hydrides andtransition metal and/or rare-earth metal catalysts to yield a productcomprising a complex alkali metal aluminum hydride; b) dehydrogenatingthe product obtained in step a) to yield hydrogen gas and at least oneother product; c) hydrogenating said other product to yield said complexalkali metal aluminum hydride according to step a); and d) repeatingsteps b) and c); said method being conducted in the absence of organicsolvents.
 2. The method according to claim 1, wherein Li, Na and/or Kmetals are used as said alkali metals.
 3. The method according to claim1, wherein LiH, NaH and/or KH are used as said alkali metal hydrides. 4.The method according to claim 3, wherein NaH is employed as said alkalimetal hydride.
 5. The method according to claim 1, wherein the molarratio between the aluminum metal and the alkali metal or alkali metalhydride is from 1:0.3 to 1:5.
 6. The method according to claim 5,wherein the aluminum metal and the alkali metal or alkali metal hydrideare employed at a molar ratio of about 1:1 to from MAlH₄ (M=Li, Naand/or K).
 7. The method according to claim 5, wherein the aluminummetal and the alkali metal or alkali metal hydride are employed at amolar ratio of about 1:3 to form M₃AlH₆ (M=Li, Na and/or K).
 8. Themethod according to claim 1, wherein said alkali and alkaline earthmetals or their hydrides are employed as finely divided powders.
 9. Themethod according to claim 8, wherein said alkali metals or theirhydrides are pretreated by milling or ultrasonication prior to use. 10.The method according to claim 1, wherein the aluminum metal is employedas a finely divided powder.
 11. The method according to claim 10,wherein said aluminum metal is optionally pretreated by heating undervacuum, ultrasonication, milling or chemical activation prior to use.12. The method according to claim 10, wherein the finely divided powderis aluminum grindings.
 13. The method according to claim 1, wherein thecatalysts are transition metals and/or transition metal compounds oralloys of Periodic Table groups 3 to 11 and of the rare-earth metals.14. The method according to claim 13, wherein the metals of saidtransition metal or rare-earth metal catalysts are bonded to elements ofPeriodic Table groups 14 to 17 or hydrogen.
 15. The method according toclaim 14, wherein said transition metal or rare-earth metal catalyst areemployed in the form of halides, hydrides, alcoholates, amides,organometallic compounds and/or intermetallic compounds or theirhydrides.
 16. The method according to claim 15, wherein titanium andzirconium are employed as said transition metals.
 17. The methodaccording to claim 13, wherein said transition metals or their compoundsare employed in amounts of from 0.1 to 10 mole percent, based onaluminum.
 18. The method according to claim 17, wherein said transitionmetals or their compounds are employed in amounts of from 1 to 5 molepercent, based on aluminum.
 19. The method according to claim 1, whereinall components of the mixture are mechanically mixed, stirred milledtogether to a first hydrogenation.
 20. The method according to claim 1,wherein said hydrogenations are performed at pressures of between 5 and150 bar and at temperatures of between 20 and 200° C.
 21. The methodaccording to claim 1, wherein said dehydrogenations are performed attemperatures of between 50 and 250° C.
 22. A process for preparing ahydrogen storage material comprising one or more alkali metal alanatesdoped with one or more transition metal or rare-earth metal catalystssaid process comprising reacting a mixture of aluminum metal with analkali metal or alkali metal hydride in the presence of said one or morecatalysts and in the absence of an organic solvent.